1. Field of the Invention
The present invention relates to a pneumatic radial tire.
2. Description of the Related Art
Pneumatic radial tires such as that described in GB Patent No. 815,055 are conventionally well known. The tire described in this patent comprises a carcass layer consisting of at least one carcass ply having a plurality of cords embedded therein and substantially perpendicular to an equatorial plane of the tire, and a belt layer disposed radially outside the carcass layer and including at least one belt ply which has a large number of cords bent in a wavy or zigzag form and which are arranged substantially in parallel to the equatorial plane of the tire.
In such a pneumatic radial tire, however, if it has run for a long period of time in a loaded condition, the cords in the belt ply, particularly the cords in the vicinity of widthwise opposite edges of the tire may be broken due to fatigue. The reason is that circumferential stretching and compressive strains may be repeatedly produced in the belt ply of the pneumatic tire by deformation due to contacting the ground during running, and such strains similarly act even on the cords embedded in the belt ply. Because the cords embedded in the belt ply are arranged substantially in parallel to the equatorial plane of the tire and bent in a wavy or zigzag form, if they are stretched and compressed circumferentially as described above, a bending moment may be produced in the bends of the filaments constituting the cord so as to develop a surface strain in the surfaces of the bends of the filaments. The smaller the radius of curvature of the bends and the larger the value of the above-described circumferential strain, the larger the value of the surface strain becomes. If this value exceeds an acceptable limit, the filaments may be broken at the bends due to fatigue. When the tire is running in a loaded condition, the circumferential strain is larger in a portion closer to the widthwise opposite edges of the belt ply and hence, the maximum surface strain may be produced at the bends of the cords at opposite edges of the belt ply, so that the cords in this portion may be broken first due to fatigue.
In addition, such a pneumatic radial tire has lower cornering power and lower steering stability when cornering. The reason is as follows: It is known that the cornering power Cp is generally proportional to the value of the Gough rigidity of the tire as represented by the following expression: ##EQU1## wherein G is an internal shearing rigidity; and each of A and B is a constant. However, since the belt ply has a smaller rigidity because of its smaller circumferential bending rigidity, i.e., its smaller value of EI, a smaller value of the cornering power Cp results.